The Unite union has written a letter to Cadbury's shareholders urging them to reject Kraft's £11.9bn offer for the confectionery firm.

Investors have until 2 February to decide whether to accept the 850p cash-and-shares offer on the table from the American food conglomerate, which owns household brands including Philadelphia and Maxwell House.

Announcing the deal on Tuesday, Cadbury's chairman, Roger Carr, conceded that there would be job losses and Unite argues that despite Kraft having boosted its original offer by 15%, the deal is "bad news" for workers and shareholders alike. It has also been criticised by the famous investor Warren Buffett, whose investment vehicle, Berkshire Hathaway, is Kraft's biggest investor with a 9.4% stake, as a "bad deal".

Unite's national officer, Jennie Formby, said: "We sincerely hope Cadbury shareholders will look before they leap into the takeover. Our huge fear is for the many thousands of workers in the UK and Ireland who depend on Cadbury for their livelihoods. They are devastated that the company and its values will change beyond recognition."

The takeover now looks an inevitability after the only possible rival bidder, Hershey, withdrew. In a statement to the stock exchange, the company confirmed that it did not intend to make a bid but stressed that it reserved the right to get involved if Kraft's offer was unsuccessful or another bidder emerged. Neither possibility seems likely. Insiders said Hershey's decision followed a unanimous vote by the company's board of directors late on Wednesday.

Kraft only needs the support of 50% of Cadbury's shareholders to win control. It is a figure that looks achievable as despite objections from British institutions such as Standard Life, 40% of its shareholders are American funds, which have no objections to owning Kraft stock. Some 20% are short-term investors such as hedge funds, which have indicated that they would accept 850p.

Unite said it had been inundated with messages from smaller shareholders who were not in favour of the deal. "The City boys and hedge funds, who are only interested in turning a quick profit and care nothing for the people or communities of Cadbury, cannot be allowed carry the day," said Formby.

The union, which is asking Kraft to promise not to close sites or erode terms and conditions on its watch, said it planned to lobby parliament to press for support from the government and MPs in its fight to protect Cadbury jobs and investment.